Mixed metal iron oxides and uses thereof

ABSTRACT

This invention is directed to novel mixed transition metal iron (II/III) catalysts for the extraction of oxygen from CO 2  and the selective reaction with organic compounds.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of 61/860,637 filed Jul. 31, 2013,Shen et al., Atty. Dkt. RTI13002USV which is hereby incorporated byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant No.DE-FE0004329 awarded by U.S. Department of Energy. The government hascertain rights in the invention.

1. FIELD OF THE INVENTION

This invention relates generally to the discovery of novel mixedtransition metal iron (II/III) catalysts for the extraction of oxygenfrom CO₂ and the selective reaction with organic compounds.

2. BACKGROUND OF THE INVENTION 2.1. Introduction

The use of CO₂ as a chemical feedstock or an oxidant is an appealingstrategy for reducing greenhouse gas emissions especially iftechnologies currently being developed to remove CO₂ from fossil fuelfired power plant exhaust gases lead to abundant, high purity, carbondioxide feedstocks. If the CO₂ gas streams can be used as reactants inprocesses which yield more energetic products, such as fuels orvalue-added intermediates, then the original carbon in the fossil fuelwould be recovered for utilization in another application. Exemplarypathways exist for converting carbon-dioxide to products which can beused in the energy industry for fuel or by the chemical industry forchemical feedstock. These include char gasification to make carbonmonoxide from carbon dioxide and carbon, carbon dioxide methanation tomake methane from carbon dioxide and hydrogen, and carbon dioxidereforming to make carbon monoxide and hydrogen from carbon dioxide andmethane. See, for example Kolb and Kolb, 1983, J Chem Ed 60(1) 57-59“Organic Chemicals from Carbon Dioxide.” Others have reported on studiesof CO₂ as a reagent for organic synthesis. See Colmenares, 2010, CurrentOrganic Synthesis 7(6) 533-542 “Novel Trends in the Utilization of CO₂as a Reagent and Mild Oxidant in the C—C Coupling Reactions.” Thepotential for the upgrading of carbon dioxide through industrialprocesses has been investigated for over the past one hundred years.

Specifically, U.S. Pat. No. 4,185,083, Walker, discloses a process usingthe Boudouard Reaction to produce finely divided carbon. U.S. Pat. No.4,496,370, Billings, and U.S. Pat. No. 4,382,915, Sadhukhan andBillings, disclose a zinc oxide-char gasification process. U.S. Pat. No.7,259,286, Jothimurugesan et al. disclose iron oxide catalysts forcarbon monoxide hydrogenation reactions such as Fischer-Tropschreaction. The contents of the above are hereby incorporated in itsentirety.

Towards these uses certain iron-based materials have been reported dueto the high reactivity of reduced iron for oxidation. For example, Tadaet al. disclose Fe-valve metal-Pt group elements (including Ru) alloysactivated by hydrofluoric acid (HF) for the conversion of CO₂ and H₂ tomethane (methanation of CO₂). Tada, et al., AMORPHOUS FE-VALVE METAL-PTGROUP METAL ALLOY CATALYSTS FOR METHANATION OF CO₂. Mater. Sci. Eng.A-Struct. Mater. Prop. Microstruct. Process. 1994, 182, 1133-1136.

Recently, Coker et al. reported iron oxide supported on zirconia oryttria-stabilized zirconia (YSZ) for the solar thermal production ofhydrogen from water or CO from CO₂. Coker et al. J. Mat. Chem 2012 226726-6732.

3. SUMMARY OF THE INVENTION

In particular non-limiting embodiments, the present invention provides amixed transition metal iron (II/III) catalyst for catalyzing CO₂oxidation of carbon or an organic compound. In one embodiment, the mixedtransition metal iron (II/III) catalyst is an iron (II/III) and atransition metal selected from the group consisting of Ag, Bi, Co, Cu,La, Mn, Sn, Sr, Ru, and Zn. The mixed transition metal iron (II/III)catalyst may further comprise a support and/or an alkali oralkaline-earth element promoter. The support may be Al₂O₃, SiO₂, TiO₂,ZrO₂ or a mixture thereof.

The mixed transition metal iron (II/III) catalyst may have the formulaFe₂O₃(SnO₂)_(0.1-10)(Al₂O₃)_(0.1-10) or the formulaFe₂O₃(SnO₂)_(1.0-3.0)(Al₂O₃)_(1.0-3.0).

Alternatively, the mixed transition metal iron (II/III) catalyst mayhave the formula (RuO₂)_(0.001-0.2)Fe₂O₃, or (RuO₂)_(0.005-0.05)Fe₂ ₃.

The invention also provides a method for converting CO₂ and carbon tocarbon monoxide which comprises contacting the mixed transition metaliron (II/III) catalyst with an appropriate CO₂ feed stream underappropriate conditions. The mixed transition metal iron (II/III)catalyst, and the appropriate CO₂ feed stream may be reacted together atthe same time in a suitable reactor such as a fluidized bed.

In another embodiment, the invention provides a method for converting ahydrocarbon to an oxygenated hydrocarbon which comprises contacting themixed transition iron (II/III) metal catalyst with the hydrocarbon andan appropriate CO₂ feed stream under appropriate conditions so as toform the oxygenated hydrocarbon. In some embodiments the catalyst may becombined with reactants which are fed simultaneously to a reaction zone.In other embodiments the catalyst itself may be transported betweenreaction zones containing separate reactant feed streams.

The hydrocarbon may be an alkane, an alkene, an alkyne, an aromaticcompound, a cyclic compound, a polyaromatic compound or a polycycliccompound. The oxygenated hydrocarbon may be an alcohol, aldehyde, ananhydride, a carboxylic acid, an ester, an ether, an epoxide, or aketone. In one embodiment, the epoxide is ethylene oxide or propyleneoxide.

The invention also provides a method for oxidative dehydrogenation (ODH)of a first hydrocarbon comprises contacting the mixed transition iron(II/III) metal catalyst with the first hydrocarbon and an appropriateCO₂ feed stream under appropriate conditions so as to form adehydrogenated second hydrocarbon. The first hydrocarbon may be analkane, an alkene, an alkyne, an aromatic compound, a cyclic compound, apolyaromatic compound or a polycyclic compound. For this method, thefirst hydrocarbon is methane and the second hydrocarbon is ethane or ahigher molecular weight hydrocarbon. For this method the firsthydrocarbon can be methane or any other saturated hydrocarbon and thesecond hydrocarbon product contains carbon atoms in which there arefewer carbon-hydrogen bonds when compared to the first hydrocarbon.

In the methods above, the alkane may be butane, ethane, methane, orpropane; the alkene may be ethylene or propylene; aromatic compound maybe ethyl benzene; or the cyclic compound may be cyclohexane.

4. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1. Removal of oxygen from carbon dioxide by a reduced ironcatalyst.

FIG. 2. Oxidation and reduction scheme in thermogravimetric experimentsand nominal catalyst formulation.

FIG. 3. Percent weight change of catalyst during thermo gravimetricanalysis (bottom) and the corresponding temperature (top) in run 1.

FIG. 4. Rate of weight change of catalyst versus temperature in run 1.The numbers denote the order of each step in the method to the left ofits extreme.

FIG. 5. Labeling of (SnO₂)(Fe₂O₃)Al₂O₃ catalyst using C¹⁸O₂.

FIG. 6. Mass spectrometry signals of relevant species over time (below),and the corresponding temperature (above).

FIG. 7. Mass spectrometry signals of relevant species during Step 4(below) and the corresponding temperature (above).

FIG. 8: Percent weight change of catalyst during thermogravimetricanalysis (bottom), and the corresponding temperature (top) in run 2.

FIG. 9: Rate of weight change of catalyst versus temperature in run 2.The numbers denote the order of each step in the method to the left ofits extreme.

FIG. 10: Proposed mechanism for metal-mediated CO₂ utilization viaconversion to CO

FIG. 11: Temperature, CO, and CO₂ profiles as a function of time—Step 1

FIG. 12: CO and CO₂ profiles as a function of temperature—Step 1

FIG. 13: Temperature, CO, and CO₂ profiles as a function of time—Step 2

FIG. 14: CO and CO₂ profiles as a function of temperature—Step 2

FIG. 15: Temperature, CO, and CO₂ profiles as a function of time—Step 3

FIG. 16: CO and CO₂ profiles as a function of temperature—Step 3

FIG. 17: Temperature, CO, and CO₂ profiles as a function of time—Step 4

FIG. 18: CO and CO₂ profiles as a function of temperature—Step 4

FIG. 19. Reduction of (RuO)_(0.1)(Fe₂O₃) with 0.2 atm CO.

FIG. 20. Oxidation of (RuO)_(0.1)(Fe₂O₃) with 1 atm CO₂.

FIG. 21. Reduction of (RuO)_(0.1)(Fe₂O₃) with 1 atm CH₄.

FIG. 22. At 500° C. (6:1CH₄:CO₂, 1 bar), there is little CO or H₂detected. When the temperature is increased to 600° C., CO and H₂ aredetected. When the pressure is raised to 25 bar, about 10 vol % CO andH₂ is detected.

FIG. 23. At 1 bar (1:1CO₂:CH₄) at 400° C. there is little synthesis gasproducts. At 500° C. there is approximately 2.5 vol % CO detected and at600° C. there is approximately 5 vol % CO and 1 vol % H₂.

FIG. 24. At 1 bar (1:1CO₂:CH₄) at 780° C. there is approximately 30 vol% CO and 10 vol % H₂.

FIG. 25. A schematic showing non-limiting examples of the catalysis of(i) CO₂+C→2CO and (ii) CO₂ used as an oxidant to produce a wide varietyof industrially important organic chemicals.

5. DETAILED DESCRIPTION OF THE INVENTION

This invention provides specific mixed-metal oxides have been developedwhich can remove an oxygen from CO₂ and utilize the oxygen for theproduction of higher-value oxygenated, or oxidized, products. In theirreduced forms, the mixed-metal oxides have been shown to remove oxygenfrom the strong carbon-oxygen bond of CO₂ (bond dissociation energy=−803kJ/mol). The mixed-metal oxide is shown to facilitate transfer of theabstracted oxygen to other substrates and is catalytic in deoxygenationand oxygen transfer. The catalyst is shown to be able to transfer theabstracted oxygen to carbon-based reductants in several oxidation statessuch as carbon (C(s), e.g. pet coke), carbon monoxide (CO), and methane(CH₄). The catalyst will be useful for utilization of CO₂ for theproduction of C₁ oxygenate from pet coke and char, for the utilizationof CO₂ as an oxygen source for selective hydrocarbon oxidations,dehydrogenations, and oxidative coupling, and for upgrading low-valuehydrocarbons to higher-value or more useful products.

Several materials have been developed which catalyze theReverse-Boudouard reaction for the production of CO from CO₂ and carbonin a reactor system operated at 800° C. We have shown conclusively thatthe catalyst materials operate by a catalyst-mediated extraction ofoxygen from carbon dioxide to the reduced catalyst surface. The removalof oxygen from CO₂ is followed by transfer of the oxygen to a differentcarbon atom, and works for carbon in reduced oxidation states such asC(0), C(−2), or C(−4), as shown in FIG. 1.

The potential for the upgrading of carbon dioxide through industrialprocesses has been investigated over the course of the past one hundredyears. Historically attractive energy applications have includedproduction of methanol from CO₂ by methane reforming (Carnol process),methane production by hydrogenation of CO₂ (Sabatier reaction), andproduction of carbon monoxide and hydrogen by reforming CO₂ withmethane. Carbon dioxide can be combined with carbon and transformed intocarbon monoxide by the Reverse-Boudouard reaction in a reaction which isthermodynamically favored at high temperature (900° C.). Severalresearchers have explored catalysts for the Reverse-Boudouard reactionin the past. Among them, some have explored the oxidation and reductionof iron on elemental carbon supports and impregnated in coal. Alkalicarbonates have also been used to catalyze char gasification by CO₂.Others have studied binary alkali-iron and alkaline-earth-iron mixedmetal oxide systems and shown them to catalyze the formation of CO fromcarbon dioxide and chars. While other mixed metal oxides with nickel,ceria, and zirconia have been recently explored for carbon dioxideutilization by reforming to synthesis gas and by methanation, mixedmetal oxides containing Group 8 metals and reducible oxides of p-blockmetals, specifically tin, have not been reported for the gasification ofcarbon with CO₂.

In one embodiment, this invention provides SnO₂Fe₂O₃Al₂O₃ as a catalystfamily for deoxygenation of CO₂ and utilization of the oxygen from CO₂with other carbon reductants to produce valuable chemicals and fuels.

The use of SnFeOx catalysts for deoxygenation of carbon oxygenates frombiomass pyrolysis vapors has been disclosed in PCT/US2013/029379, thecontents of which are hereby incorporated in its entirety.

This invention disclosure covers quaternary and even quintenaryvariations of the Fe₂O₃(SnO₂)Al₂O₃ catalyst formulation for CO₂utilization. The most obvious additives are alkali and alkaline-earthmetal promoters which can be added by many salt forms. Many variationswere discovered, formulated, tested, and shown to work during thisstudy.

This invention disclosure also covers a broad range of iron to tin toaluminum in the catalyst formulation, intended as all feasible ratios.Many variations were discovered, formulated, tested, and shown to workduring this study.

This invention disclosure covers any formulation involvingFe₂O₃(SnO₂)Al₂O₃ calcined under all feasible calcination conditions. Thecatalysts may be useful for CO₂ utilization for CO production, chargasification, and selective oxidations of hydrocarbon reductants,oxidative methane coupling, oxidative dehydrogenation of light alkanesfor olefin production, epoxidation of olefins to prepare alkene-oxides,preparation of methanol and dimethyl ether synthesis. The reagentsdisclosed herein may be used to produce additional commerciallyimportant products including but not limited to, acetic acid, aceticanhydride, ethylene vinyl acetate (EVA), styrene, terephthalic acid,formic acid, n-butanal, 2-methylpropanal, acrylic acids, neopentylacids,propanoic acid, dimethyl formamide, and Fischer-Tropsch hydrocarbons.

These important industrial materials can be used to manufacture avariety of finished goods, e.g., EVA for adhesives, glues, plastics, andfoam rubber. EVA based consumer products include hot melt adhesives,glue sticks, plastic wraps, foam rubber, floats, fishing rods, shoes,and photovoltaics.

5.1. Compositions

As used herein the term “mixed transition metal iron (II/III) catalyst”means Fe⁺² or Fe⁺³ mixed with a second metal which may be (i) a d-blockelement, IUPAC Groups 3-12; (ii) a “post-transition” metal (Al, Ga, In,Sn, Tl, Pb, Bi, Po); or an f-block element such as a lanthanide oractinide, sometimes referred as to as an “inner transition metal”; or acombination of (i), (ii) or (iii). The term mixed transition metal iron(II/III) catalyst includes the reagents disclosed herein. The termincludes various oxidized forms of Fe including reactive speciesgenerated in situ such as Fe⁰ or Fe⁺¹ in the catalyst. Mixed transitionmetal iron (II/III) catalysts are ionic materials; that is, they arematerials that no longer retain metallic characteristics unlike metalalloys.

The invention provides compositions for the mixed transition metal ironoxide (II/III) catalysts. A non-limiting diagram of just some of theuses of the catalysts in shown in FIG. 25. The compositions can bedescribed according to the formula ABCD, where each alphabetical letterindicates a set of metal oxides or mixed metal oxides from which one isselected and used with a member of another set. As few as two sets maybe used, such as AC, BC, or DC. Also three sets may be used, such asACD, ABC, or BCD. All four sets may be used, such as ABCD. Set C is onlyinclusive of iron.

The mixed transition metal may be a group A component, as exemplified bySnO₂. The group A component is involved in oxygen transport and CO₂oxygen extraction. The group A components may also be: BaCoO₃, Bi₂O₃,CaOZrO₂, CeO₂, Gd₂O₃, Gd₂Zr₂O₇, GdTi₂O₇, La_(1-y)Sr_(y)CoO_(x),La_(1-y)Sr_(y)Ga_(1-z)Mg_(z)O_(x), La₂O₃, LaAlO₃, LaGaO₃, MgOZrO₂,Nd₂Zr₂O₇, NdGa_(1-y)Mg_(y)O_(x), NdGaO₃, SmTi₂O₇, SrCoO₃, Y₂O₃ZrO₂,YTi₂O₇, or ZrO₂.

Alternatively, the mixed transition metal may be a group B component,exemplified by RuO₂ and metal oxides. The group B components areinvolved in CO₂ oxygen extraction and hydrocarbon selective/partialoxidation. The group B components may also be: AgO₂, Co₂O₃, CuO,La_(1-y)Sr_(y)CoO_(x), La_(1-y)Sr_(y)O_(x), Mn₂O₃, Mn₂O₇, Mn₃O₄, MnO,MnO₂, MoO₃, Re₂O₇, or V₂O₅.

The group C component is exemplified by Fe₂O₃. The group C component isinvolved in oxygen transport and CO₂ oxygen extraction. The group Dcomponent is a support for the mixed transition metal iron catalystswhich is exemplified by Al₂O₃. One of ordinary skill in the art wouldrecognize additional supports. The components of group D may be Al₂O₃,Al₂O₃—SiO₂, CaAl₂O₄, CaOZrO₂, K₂Al₂O₄, MgAl₂O₄, MgOZrO₂, Na₂Al₂O₄, SiO₂,TiO₂, Y₂O₃ZrO₂, or ZrO2. Other, non-catalyst heat transfer media alsocan be used, such as alumina, silica, olivine, and sands.

Furthermore, the catalysts may also include a promoter which will act tolower the work function or suppress sintering and/or coking. Thepromoter components may be a compound having the formula A₂O; A₂CO₃; orA(OH) (where A=Na, K, Cs); BO; BCO₃; B(OH)₂ (where B═Mg, Ca, Sr); or amixture of A and B compounds.

In one embodiment, the mixed transition metal iron (II/III) catalyst mayhave the formula Fe₂O₃(SnO₂)_(0.1-10)(Al₂O₃)_(0.1-10). In alternativeembodiments, the mixed transition metal iron (II/III) catalyst may havethe formula Fe₂O₃(SnO₂)_(0.2-5.0)(Al₂O₃)_(0.2-5.0),Fe₂O₃(SnO₂)_(1.0-2.0)(Al₂O₃)_(0.5-5.0), Fe₂O₃(SnO₂)_(0.5-5.0)(Al₂O₃)_(1.0-3.0),Fe₂O₃(SnO₂)_(1.0-3.0)(Al₂O₃)_(1.0-3.0),Fe₂O₃(SnO₂)_(1.0-2.5)(Al₂O₃)_(1.0-2.5), orFe₂O₃(SnO₂)_(1.2-2.2)(Al₂O₃)_(1.2-2.2).

The mixed transition metal iron (II/III) catalyst may have the formula(RuO₂)_(0.001-0.2)Fe₂O₃. Alternatively, it may have the formula(RuO₂)_(0.002-0.1)Fe₂O₃, (RuO₂)_(0.005-0.05)Fe₂O₃,(RuO₂)_(0.008-0.02)Fe₂O₃, (RuO₂)_(0.01-0.02)Fe₂O₃.

Table 1 shows compounds that were prepared and their reactiontemperatures.

Reduction Reduction Temperature Temp. Reduction Oxidation Range RangeCapacity Temp. Metal Oxide (H₂, ° C.) (CO, ° C.) (CO, wt %) (CO₂)La₂O₃SrOCoO Fe₂O₃ 250-550 400-490  4 200-500 MnO₂Fe₂O₃ 400-450 300-45011.7 250-450 (K)_(0.1)((Mg)_(0.1)((CuO)_(0.38) 300-370 150-550,  6.7350-450 (Fe₂O₃)_(0.29)(Al₂O₃)_(0.33))) 700-800 (CuO)0.38(Fe2O3)0.29150-230 100-400  8.8 — (Al2O3)0.33 (RuO₂)_(0.024)Fe₂O₃ 225-265, 250-42519.9 350-450 350-775 (RuO₂)_(.049)Fe₂O₃ 225-270, 225-425 19.3 350-450400-850 (RuO₂)_(.012)Fe₂O₃ 230-290, 230-400 21.2 360-450 400-900RuO_(0.024)Fe₂O₃ 225-245, 225-425, 16.3 350-450 400-(Fe₂O₃)_(0.56)(SnO₂)_(0.78)Al₂O₃ 225-400, 16.8 600-800 475-800 225-400,(K)_(.001)(Mg)_(.0025)(Fe₂O₃)_(0.56)(SnO₂)_(0.78)Al₂O₃ 590-650 475-80016.8 650-800 (K)_(0.15)(Mg)_(0.1275)((Fe₂O₃)_(0.56) 550-725 525-800 12400-775 (SnO₂)_(0.78)Al₂O₃)_(0.7225) (MnO₂)_(0.2)(ZnO₂)_(0.8)Fe₂O₃600-675 250-450 15 650-700

The reduction temperatures are the range of temperatures at which thematerials can be reduced by hydrogen gas or carbon monoxide gas to makereactive reduced catalysts. The reduction capacity is the percentage ofthe mass which is decreased by the removal of oxygen from the catalyst.The oxidation temperature is the temperature range in which the reducedmaterial is reoxidized by carbon dioxide.

The catalytic reaction can be carried out in a variety of differenttypes of reactors. Preferably, the reactor is a fluid-type reactor, suchas a fluidized bed or a transport reactor. In one embodiment, a riserreactor may be used. The CO₂ and carbon and/or organic startingmaterials may be provided to the reactor at a defined rate—e.g., a ratesuch that the residence time is less than defined time, such as about 5seconds or less.

Preferably, the reactor used is one that is capable of achieving thenecessary conditions to form a specific reaction product. Specifically,it can be beneficial to use a reactor that is adapted for relativelyshort residence times of the reactants and the catalyst in the reactor,as noted above.

Another condition to be considered is reaction temperature. In specificembodiments, the reacting of the CO₂ and carbon and/or organic startingmaterials in the presence of the catalyst can be carried out at atemperature of about 200° C. to about 900° C., about 300° C. to about700° C., about 350° C. to about 600° C., about 400° C. to about 500° C.or a temperature of about 550° C. or less. In other embodiments, thereacting of the CO₂ and carbon and/or organic starting materials can becarried out at a pressure of up to about 25 bar (2.5 MPa) or about 80bar (8.0 MPa). In some embodiments, reacting can be carried out atambient pressure to near ambient pressure.

The process of the disclosure can comprise separation of the productsinto two or more different fractions. This can comprise transferring thestream comprising the product(s) to a separator. In some embodiments,the stream may be separated into a vapor and gas fraction and a solidsfraction, which comprises solid reaction products and the catalyst. Theinventive method also can comprise regenerating and recycling thecatalyst into the pyrolysis process. In some embodiments, this also mayinclude transferring the catalyst from the separator through a reducingzone prior to re-introduction into the reactor.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. The article “a” and “an” areused herein to refer to one or more than one (i.e., to at least one) ofthe grammatical object(s) of the article. By way of example, “anelement” means one or more elements.

Throughout the specification the word “comprising,” or variations suchas “comprises” or “comprising,” will be understood to imply theinclusion of a stated element, integer or step, or group of elements,integers or steps, but not the exclusion of any other element, integeror step, or group of elements, integers or steps. The present inventionmay suitably “comprise”, “consist of”, or “consist essentially of”, thesteps, elements, and/or reagents described in the claims.

It is further noted that the claims may be drafted to exclude anyoptional element. As such, this statement is intended to serve asantecedent basis for use of such exclusive terminology as “solely”,“only” and the like in connection with the recitation of claim elements,or the use of a “negative” limitation.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimits of that range is also specifically disclosed. Each smaller rangebetween any stated value or intervening value in a stated range and anyother stated or intervening value in that stated range is encompassedwithin the invention. The upper and lower limits of these smaller rangesmay independently be included or excluded in the range, and each rangewhere either, neither or both limits are included in the smaller rangesis also encompassed within the invention, subject to any specificallyexcluded limit in the stated range. Where the stated range includes oneor both of the limits, ranges excluding either or both of those includedlimits are also included in the invention.

The following Examples further illustrate the invention and are notintended to limit the scope of the invention. In particular, it is to beunderstood that this invention is not limited to particular embodimentsdescribed, as such may, of course, vary. It is also to be understoodthat the terminology used herein is for the purpose of describingparticular embodiments only, and is not intended to be limiting, sincethe scope of the present invention will be limited only by the appendedclaims.

6. EXAMPLES 6.1. Mixed Tin Iron Oxides for Carbon Dioxide Utilization

The use of CO₂ as a chemical feedstock is an appealing strategy forreducing greenhouse gas emissions especially if technologies currentlybeing developed to remove CO₂ from fossil fuel fired power plant exhaustgases lead to abundant, high purity, carbon dioxide feedstocks.¹ If theCO₂ gas streams can be used as reactants in processes which yield moreenergetic products, such as a fuel or value-added intermediate, then theoriginal carbon in the fossil fuel would be recovered for utilization inanother application.^(2, 3) The potential for the upgrading of carbondioxide through industrial processes has been investigated for the pastone hundred years.⁴ Historically attractive energy applications haveincluded methane production by hydrogenation of CO₂ (Sabatier reaction),production of carbon monoxide and hydrogen by reforming CO₂ with methane(dry methane reforming), production of methanol from CO₂ by methanereforming (Carnol process), and gasification of chars using CO₂ to makeCO (Reverse-Boudouard reaction).⁵⁻⁸

In the Reverse-Boudouard reaction, the transformation becomesthermodynamically favoured beginning at ˜700° C. but conversion is lowbelow ˜900° C. Several researchers have explored catalysts for theReverse-Boudouard reaction in the past and have been reviewed by severalauthors.^(4, 9-21) The goal of catalysis is to increase the reactionrate at lower temperature. Among them, some have explored the oxidationand reduction of iron on elemental carbon supports and impregnated incoal using techniques such as thermogravimetric analysis, ¹³CO₂ pulsedreactions, and temperature programmed desorption.^(13, 15, 22, 23)Alkali carbonates have also been found to catalyse char gasification byCO₂ and some researchers have studied binary alkali-iron andalkaline-earth-iron mixed metal oxide systems and shown them to catalysethe formation of CO from carbon dioxide and chars.²⁴⁻³⁰ Recently mixedmetal oxides with nickel, ceria, and zirconia have been explored forcarbon dioxide utilization by reforming to synthesis gas and bymethanation.³¹⁻³³ To our knowledge, mixed metal oxides containing Group8 metals and reducible oxides of p-block metals, specifically tin, havenot been reported for the gasification of carbon with CO₂. However,until now, little work has been done to show conclusively that theoxygen extracted from CO₂ by the catalyst materials results in transferof the extracted oxygen to an external carbon source rather thanincorporation of the oxygen into the catalyst structure. We havedeveloped mixed metal oxides of tin and iron which catalyze theReverse-Boudouard reaction for production of CO from carbon feedstockssuch as pet-coke and biomass char. In this disclosure we characterizethe removal of oxygen from CO₂ by a reduced tin-iron catalyst and showthat the oxygen comes from carbon dioxide and is transferred to othercarbon sources as shown in FIG. 1. The reaction was studied usingisotopically-labeled C¹⁸O₂, thermogravimetric analysis, and massspectroscopy. The results show that the highly stable carbon-oxygenbonds of CO₂ can be broken with subsequent transfer of the oxygen to acarbon atom of lower oxidation state. One of ordinary skill will be ablemodify the catalyst formulations disclosed herein to make formulationswhich abstract oxygen from CO₂ at lower temperatures and catalysts whichcan selectively or partially oxidize other carbon-based reductantsleading to higher-value products. Furthermore, the utilization of CO₂ tofeed oxygen to catalysts in partial-oxidation processes which currentlyuse oxygen separation units has the potential to lower capital costs forprocesses while providing additional markets for captured carbon dioxidebeyond conventional enhanced oil recovery applications.

Mixed metal oxides containing tin are composed of tin-oxide phases whichare known to have temperature-induced oxygen mobility.^(34, 35) Inconsidering the SnO₂Fe₂O₃Al₂O₃ catalyst formulation and the givenreaction conditions, it is sensible to question what types of oxygencontaining sites are involved in the reduction of carbon dioxide and toconsider the extent of oxygen transfer synergies. One simplisticperspective is to consider the oxygen in the catalyst associated withSnO₂ as distinct from the oxygen which is associated with Al₂O₃ andlikewise for the oxygen associated with Fe₂O₃. The nominal formulationof the catalyst investigated here is (Fe₂O₃)(SnO₂)_(1.41)(Al₂O₃)_(1.82)and is given in FIGS. 1 and 2 along with a proposed mechanism whichbroadly describes a pathway for oxygen transfer. This mechanistichypothesis can be tested by Thermogravimetric analysis (TGA). Forexample, the weight loss observed for loss of oxygen exclusively fromSnO₂ (8.1% theoretical observed for all the oxygen in the catalyst). Thetheoretical limit to the weight loss due to complete oxygen loss is32.4%. Intermediate weight losses could correspond to loss of oxygenfrom a combination of SnO₂ and Fe₂O₃ (16.7%), or only Fe₂O₃ (8.6%), oreven by incomplete reduction of Fe₂O₃ to FeO (5.7%).

Since the thermogravimetric analyses shown in FIG. 3 shows the totalweight loss to be only approximately 21.6%, it is not possible that allthe oxygen in the materials is available to reduction. Similarly, sincean overall weight loss of 21.6% is observed starting from ambient, it isnot likely that the oxygen originates exclusively from SnO₂ orexclusively from Fe₂O₃ given the elemental analysis (SupplementaryInformation, Sec. 6.3. below). FIG. 3 shows that the overall weight lossobserved from ambient is approximately 21.6%. Approximately 7.4% of theinitial weight is lost upon heating the sample to 800° C. in nitrogen(dark grey, inert). Presumably, this corresponds to loss of surfaceadsorbed and absorbed species such as adventitious water, oxygen, orcarbon dioxide. Further changes in weight are described relative to thesample weight following the initial desorption as suggested by the fourhorizontal lines on the weight profile.

Following the inert thermal ramp, the weight of the sample is furtherdecreased when the material is again heated to 800° C. in the presenceof 10% CO (N₂ balance, white). The weight loss due to reduction by CO isapproximately 15.4%. Subsequent oxidation with CO₂ results in a weightgain of about 99.1% of the previous weight loss (light grey). Followingthe treatment with CO₂ about 0.5% of the initial weight is lost byramping to 800° C. in nitrogen. When the catalyst is again treated withCO in a second reduction step, a smaller weight loss (˜13.3%) isobserved compared to the first reduction step. This is consistent withirreversible transition from mixed valent Fe₂O₃ to lower valent Fe₃O₄, atransition which accounts for approximately 3.4 wt % change due tooxygen loss. It is also consistent with the hypothesis that somecatalyst is lost to deactivation, either reversible, or irreversible.One reversible catalyst deactivation route is the forward BoudouardReaction, where one equivalent carbon is deposited from thedisproportionation of two equivalents of CO. A follow-up oxidation stepleads to a weight gain equal in magnitude to the weight loss observedduring the previous reduction. A slight weight gain is then observedwhen the oxidized catalyst is further oxidized while heated to 800° C.in air, returning the sample to approximately the same weight observedafter the initial desorption. After air oxidation, reduction with COshows a 14.0% weight loss.

In a follow-up experiment (Supplementary Information, Sec. 6.3 below),the catalyst was reduced again with CO after two cycles then oxidizedwith air. It showed a return to the weight observed prior to allreduction steps and at the end of each oxidation step. This comparisonshows that the catalyst can obtain oxygen from CO₂, a relatively pooroxidant, almost as effectively as it can from O₂, a relatively strongoxidant.

Overall, the weight changes observed in the thermogravimetric analysesin the absence of a reductant are most likely due to desorption ofadventitious adsorbates (H₂O, CO₂, possibly O₂) from the surface of thecatalyst. In the presence of a reductant, both SnO₂ and Fe₂O₃ sites arereduced when heated to 800° C., but Al₂O₃ sites do not appear to bereduced. The observed weight loss (15.5%), agrees well with the amountof oxygen calculated to be associated with SnO₂ and Fe₂O₃ (16.7%).

It must be noted that the thermogravimetric analysis cannot be used toconclusively rule out coincidental weight changes resulting fromcombinations of partial oxygen losses from SnO₂, Fe₂O₃, and Al₂O₃ sites.However, FIG. 4 shows plots of the observed weight changes withtemperature purge gas over the temperature range from 0-800° C. Eachweight change trace is numbered for distinction. The derivative plotsindicate that changes in weight are likely due to three events andinvolve two types of active catalytic sites. The weight change observedduring the initial temperature ramp in nitrogen (black) peaks distinctlyat 100° C. in agreement with the hypothesis that the initial weight lossinvolves the loss of adventitious absorbates. When the reduced catalystis oxidized by treatment with CO₂ (dashed traces), two separate eventsare observed to occur, the first at approximately 650° C. and the secondoccurring at approximately 720° C. The bimodal distribution for weightchange under oxidizing conditions is reproducible in both CO₂ treatmentsteps. These observations are consistent with oxygen abstraction fromCO₂ occurring at two different sites, one active at slightly lowertemperature than the other. In the reduction steps, a bimodaldistribution is also observed. A low temperature weight change isobserved at approximately 400° C. and is minor compared to the highertemperature weight change observed at 700° C. A third minor weightchange is also observable above 700° C. but is not as pronounced as theprimary peak. It is also observed that in the initial reduction cycle,weight changes are observed at slightly lower temperatures compared tothe next two cycles. This could indicate that an irreversible transitionis made on the active site during the first reduction. This would beconsistent with the transition of Fe₂O₃ to Fe₃O₄, with the reduction ofmixed oxide Fe (III)/Fe (II) anticipated to be easier than the reductionof a more reduced Fe(II)/Fe(III). Lastly, the sample is treated with air(-- -) after the catalyst has been oxidized with CO₂, and there islittle observable change in weight during this event.

Mass spectroscopy (MS) experiments were conducted with isotopicallylabelled C¹⁸O₂. The study reveals details about both the fate of theoxygen abstracted from CO₂ as well as the capability of the catalyst totransfer metal-oxide-associated oxygen to external carbon sources.Details of the experiment are provided in the Supplementary Information(Sect. 6.3). In short, gas exiting a fixed-bed catalyst zone wasanalysed by MS. Isotopically-labelled C¹⁸O₂, was used to follow oxygenthrough the reaction and shows the original molecular connectivity andthe molecular connectivity of the products. In the presence of acatalyst which abstracts oxygen from carbon dioxide, heavy oxygen (¹⁸O)will be removed and C¹⁸O will be produced as the primary product. Weanticipated observing this by MS upon treatment of the reduced catalystwith C¹⁸O₂. It was surmised that this would label the reduced catalystwith ¹⁸O and that the labelled catalyst could then be reduced again withCO with the resulting production of C⁶O¹⁸O as shown in FIG. 5.

The experimental results are shown below in FIG. 6 with the observedmass signals (lower) correlated to the reactor temperature (upper) andpurge gas (by shade). The most relevant segments are the first oxidationsegment (light grey), where oxygen is removed from C¹⁸O₂, and the lastreduction section (white), where CO removes oxygen from the catalyst.FIG. 6 shows that initial reduction with CO (first white segment) givesan increase in mass 44 at approximately 800° C. which corresponds toproduction of CO₂ using unlabelled oxygen from the catalyst. In thereduced state, the catalyst removes oxygen from C¹⁸O₂ at approximately650° C. as shown. The decrease in the signal intensity of C¹⁸O₂ isaccompanied by corresponding increases in C¹⁸O, but also unexpectedly byan increase in CO¹⁸O, which indicates three potential scenarios. One,labelled oxygen is coupled to surface bound CO from the previous step.Second, unlabelled oxygen from the catalyst surface is coupled to C¹⁸Obefore it is dissociated from the catalyst surface. Third, carbon isdeposited on the catalyst surface during the preceding reduction stepand gets coupled to one unlabelled oxygen and one labelled oxygen. Allthree scenarios require removal of ¹⁸O from C¹⁸O₂ and show thecapability of the material to utilize oxygen from carbon dioxide.

In the fourth step, the labelled catalyst was again treated with flowing20% CO. Here we anticipated observing a decrease in CO and acorresponding increase in CO¹⁸O associated with reduction of thecatalyst by removal of ¹⁸O which the catalyst abstracted from C¹⁸O₂.Indeed there is this correlation, however, as FIG. 7 shows, we alsoobserved the formation of all the other species which can be proposed toinvolve surface bound oxygen (CO¹⁸O, CO₂, C¹⁸O, C¹⁸O₂). The observationof all species correlated to the decrease in the CO signal. Theappearance of CO¹⁸O supports the hypothesis that heavy oxygen (¹⁸O) isabstracted from C¹⁸O₂ by the catalyst, and then added to a differentcarbon source, in this case carbon monoxide, to produce partiallylabelled carbon dioxide (CO¹⁸O). The observation of CO₂ may be primarilyfrom incomplete labelling of the catalyst in the prior oxidation step.The time dependent decrease in signal intensity for CO¹⁸O but not CO₂ isconsistent with consumption of labelled oxygen from the surface of thecatalyst over time. The observation of C¹⁸O was unanticipated, but showsthat carbon monoxide may undergo disproportionation to carbon and carbondioxide with carbon deposition on the catalyst surface where it picks upan ¹⁸O from the labelled catalyst. It is conceivable that carbonmonoxide is absorbed on the surface of the catalyst, deoxygenated, andthen reoxygenated with a labelled ¹⁸O. The detection of C¹⁸O₂ can onlybe accounted for by mechanistic routes which involve labelling of thecatalyst with ¹⁸O in the previous oxidation step followed by transfer ofthe labelled oxygen during the subsequent reduction step, either to acarbon which is absorbed by the catalyst as CO before undergoing oxygenmetathesis and oxygen addition, or to a carbon which is deposited on thecatalyst as elemental carbon before undergoing two oxygen additions withlabelled ¹⁸O which must have originated from labelled C¹⁸O₂.

In summary, the mechanistic investigation of the CO₂ utilizationcatalyst Fe₂O₃(SnO₂)_(1.41)(Al₂O₃)_(1.82) has been conducted and resultsobtained from mass spectroscopy experiments using isotopically-labelledcarbon dioxide prove that the reduced catalyst abstracts oxygen fromcarbon dioxide and transfers it to another carbon. Thermogravimetricevidence suggests that oxygen from Fe₂O₃ and SnO₂ are mobile and able tobe removed from the catalyst by reductant. Rapid exchange of oxygen bythe catalyst easily occur due to the observed high mobility of oxygenbetween the catalyst and carbon dioxide which may lead to potential sidereactions.

6.2. References for Section 6.1 (Mixed Tin Iron Oxides)

-   1. P. Markewitz, W. Kuckshinrichs, W. Leitner, J. Linssen, P.    Zapp, R. Bongartz, A. Schreiber and T. E. Muller, Energy &    Environmental Science, 2012, 5, 7281-7305.-   2. M. B. Ansari and S.-E. Park, Energy & Environmental Science,    2012, 5, 9419-9437.-   3. N. A. M. Razali, K. T. Lee, S. Bhatia and A. R. Mohamed,    Renewable & Sustainable Energy Reviews, 2012, 16, 4951-4964.-   4. M. Halmann and M. Steinberg, Greenhouse Gas Carbon Dioxide    Mitigation Science and Technology, Lewis Publishers, Washington,    D.C., 1999.-   5. K. Nagase, T. Shimodaira, M. Itoh and Y. Zhemg, Physical    Chemistry Chemical Physics, 1999, 1, 5659-5664.-   6. M. Steinberg, Brookhaven National Lab, Upton, N.Y., December    1995.-   7. S. K. Hoekman, A. Broch, C. Robbins and R. Purcell, International    Journal of Greenhouse Gas Control, 2010, 4, 44-50.-   8. F. Fischer and H. Tropsch, Brennst. Chem., 1928, 9, 29-46.-   9. S. Yokoyama, K. Miyahara, K. Tanaka, I. Takakuwa and J. Tashiro,    Fuel, 1979, 58, 510-513.-   10. T. Suzuki, H. Ohme and Y. Watanabe, Energy and Fuels, 1994, 8,    649-658.-   11. F. Carrasco-Marin, J. Rivera-Utrilla, E. U. Hidalgo and C.    Moreno-Castilla, Fuel, 1991, 70, 13-16.-   12. A. P. Dhupe, A. N. Gokarn and L. K. Doraiswamy, Fuel, 1991, 70,    839-844.-   13. H. Ohme and T. Suzuki, Energy and Fuels, 1996, 10, 980-987.-   14. F. Akiyama, Chemistry Letters, 1997, 643-644.-   15. T. Kodama, S. Miura, T. Shimuzu, A. Aoki and Y. Kitayama, Abstr.    4th Int. Conf. on Carbon Dioxide Utilization, Kyoto, Japan, 1997.-   16. R. T. Yang and C. Wong, Journal of Catalysis, 1983, 82, 245-251.-   17. K. J. Huttinger and O. W. Fritz, Carbon, 1991, 29, 1113-1118.-   18. H. Ono, M. Kawabe, H. Amani, M. Tsuji and Y. Tamaura, Abstracts    of the Fourth International Conference on Carbon Dioxide    Utilization, Kyoto, Japan, September, 1997, P-004.-   19. M. Steinberg and Y. Dong, Abstracts of the International    Conference on Carbon Dioxide Utilization, Bari, Italy, September    1993.-   20. M. Steinberg, Abstracts of the Third International Conference on    Carbon Dioxide Utilization, Norman, Oklahoma, May, 1995.-   21. B. J. Wood and K. M. Sancier, Catalysis Reviews-Science and    Engineering, 1984, 26, 233.-   22. T. Suzuki, K. Inoue and Y. Watanabe, Energy and Fuels, 1988, 2,    673.-   23. T. Suzuki, K. Inoue and Y. Watanabe, Fuel, 1989, 68, 626.-   24. J. M. Saber, J. L. Falconer and L. F. Brown, Fuel, 1986, 1356.-   25. J. M. Saber, J. L. Falconer and L. F. Brown, Journal of the    Chemical Society, Chemical Communications, 1987, 445.-   26. S. R. Kelemen and H. Freund, Carbon, 1985, 23, 723.-   27. S. Yokoyama, K. Miyahara, K. Tanaka, J. Tashiro and I. Takakuwa,    Journal of the Chemical Society of Japan, 1980, 6, 974.-   28. T. Suzuki, M. Mishima and Y. Watanabe, Chemistry Letters, 1982,    985.-   29. J. Carrazza, W. T. Tyose, H. Heinemann and G. A. Somorjai,    Journal of Catalysis, 1985, 96, 234.-   30. Y. Ohtsuka, K. Hosoda and Y. Nishiyama, Journal of the Fuel    Society of Japan, 1987, 66, 1031.-   31. M. B. Gawande, R. K. Pandey and R. V. Jayaram, Catalysis Science    and Technology, 2012, 2, 1113-1125.-   32. P. Kumar, Y. Sun and R. O. Idem, Energy and Fuels, 2008, 22,    3575.-   33. F. OCampo, B. Louis and A. Roger, Applied Catalysis a-General,    2009, 369, 90.-   34. J. Maier and W. Gopel, Journal of Solid State Chemistry, 1988,    72, 293-302.-   35. J. Mizusaki, H. Koinuma, J.-I. Shimoyama, M. Kawasaki and K.    Fueki, Journal of Solid State Chemistry, 1990, 88, 443-450.

6.3. Supplemental Information on Mixed Tin Iron Oxides for CarbonDioxide Utilization

Synthesis of Fe₂O₃(SnO₂)_(1.41)(Al₂O₃)_(1.82) Catalyst

The mixed oxide catalyst was obtained by co-precipitation of metal saltsfrom aqueous solutions using conventional procedures. Tin (IV) chloride,pentahydrate (Sigma Aldrich, 98%), iron (III) nitrate, nonahydrate(Sigma Aldrich, ≧98%), aluminum nitrate, nonahydrate (Sigma Aldrich,≧98%) and ammonium hydroxide (BDH Aristar, 28-30%), were obtained andused as received without further purification.

The catalyst was prepared according to the following procedure: 172.24 g(0.491 mole) SnCl₄.5H₂O, 281.24 g (0.696 mole) Fe(NO₃)₃.9H₂O, and 476.81g (1.271 mole) Al(NO₃)₃.9H₂O were dissolved into a beaker containing1620 g of deionized H₂O by mixing for at least 1 hour. The salt solutionwas added at a constant rate of 30 mL/min to a tank containing 1500 g ofDI water. A solution of NH₄OH (504.07 g, 4.17 mole) in DI H₂O was addedat a variable rate of 8-10 mL/min to maintain the pH of theprecipitation at 8.0±0.2. The precipitation was stopped when all themetals salts were added to the precipitation tank and the pH was equalto 8.0. The precipitation was allowed to mix for an additional 45minutes. The precipitate was filtered into two wet cakes and then washedwith DI water until the eluent contained chloride ion, as detected by asolution of 0.1M Ag(NO₃)₂, at a ppb level (based on K_(sp)). An LOI ofeach cake was used to determine the solid metal oxides content of eachcake. By calculation, 195.3 g solids were obtained, >99% yield.Elemental analysis by ICP-MS showed Fe 18.7%, Sn 28.0%, Al 16.6%, theoryFe 20.3%, Sn 30.2%, Al 17.4%.

Thermogravimetric Analysis

Thermogravimetric analysis (TGA) was conducted using TA Instruments TGAQ500 with Advantage for Q Series software. The plumbing of the TGAfurnace was altered to receive gas for the sample purge from externalmass flow controllers (MFCs), operated via an electronic control box.This allows for the selection of additional gases for the sample purgecompared to the standard Q500 design. Switching between gases wasperformed manually via in-line two-way valves, and flows were setaccording to MFC calibrations for each gas. Two temperature programswere used involving multiple steps to demonstrate the addition andremoval of oxygen from the surface of the catalyst. For each analysis, afresh sample (20-30 mg) was loaded in a tared, platinum TGA pan at thestart of the program. Each program extended over multiple days, and thesame sample was used for the duration. When necessary, the sample washeld overnight or over-weekend in the closed TGA furnace under nitrogenat room temperature. In short, both programs describe heating the sampleto 800° C. and soaking for 60 minutes before cooling back down to 30° C.using different gases to observe reducing, oxidizing, or purely thermaleffects. In both programs, two cycles of the following steps are carriedout. Thermal desorption is first observed followed by reduction, thenoxidation with CO₂, again thermal desorption, then reduction, andoxidation with CO₂. In one program, the final oxidation with CO₂ isfollowed by oxidation with air, to observe any sites which may require astronger oxidant than CO₂. In the second program, the second oxidationwith CO₂ is followed by another reduction step, then oxidation with air,to confirm that the weight gain from the reduced sites oxidized in airis the same as the weight gain observed for oxidation of the reducedsites by carbon dioxide.

AutoChem-MS Analysis with Isotopically—Labelled Gases

A Micromeritics' AutoChem II 2920 Chemisorption Analyzer was interfacedwith a Dycor Quadrupole Mass Spectrometer and used to follow thetransformations of carbon dioxide, carbon monoxide, and oxygen. TheAutoChem II 2920 is a fully automated instrument capable of conductingprecise chemical adsorption and temperature programmed reaction studies.The sample is contained in a quartz reactor housed in a clamshellfurnace, programmable up to 1100° C. Four gas inlets withhigh-precision, independently calibrated mass flow controllers provideaccurate delivery of up to four analysis gases over the course of anexperiment. For these experiments, the AutoChem was operated withconstant flow of analysis gas through the sample reactor. Gases employedwere ultra-high purity helium, a certified mixture of 20% CO in helium,and either ¹³C or ¹⁸O labelled CO₂. The Isotopically-labelled gases werepurchased from Sigma-Aldrich and used as received. Experimentalconditions for an exemplary experiment are given in Table 2 below. Theresults are given in Results and Discussion Section below.

TABLE 2 Exemplary parameters for SnO₂Al₂O₃(Fe₂O₃)₃ testing for ¹²C¹⁸O₂oxygen abstraction. Temperature Hold Temp 1 Temp2 Ramp Rate Flow TimeStep (° C.) (° C.) (° C./min) Gas (mL/min) (min) 1 40 40 0 CO/He 15 5 240 800 10 CO/He 15 5 3 800 40 50 CO/He 15 5 4 40 40 0 N₂ 15 5 5 40 40 0¹²C¹⁸O₂ 15 5 6 40 800 10 ¹²CO₂ 15 5 7 800 40 50 ¹²CO₂ 15 5 8 40 40 0 N₂15 5 9 40 800 10 N₂ 15 5 10 800 40 50 N₂ 15 20

Treatment of Reduced Catalyst with Air

In a thermogravimetric experiment described herein, a program was usedto evaluate the weight loss and weight gain shown by(Fe₂O₃)(SnO₂)_(1.41)(Al₂O₃)_(1.82) when it was heated to 800° C. whilebeing reduced with 10% CO (white) followed by oxidation with 100% CO₂(light grey). After two cycles the catalyst was reduced again with 10%CO (white), and then oxidized with air (lighter grey). The weight of thereduced catalyst after oxidation with CO₂ was the same as the weight ofthe reduced catalyst after oxidation with O₂. Experimental results ofthe experiment are given in FIG. 8.

Plots of the observed weight changes with temperatures corresponding tothe experiment described above are shown in FIG. 9. The data isdisplayed by purge gas over the temperature range from 0-800° C., thusfor most weight changes observed when ramping to 800° C. there is acorresponding static weight observation for cooling from 800° C. Eachweight change trace is numbered to indicate that it is associated with adifferent step in the TGA program. The derivative plots indicate thatchanges in weight are likely due to three events and involve two typesof active catalytic sites. The weight change observed during the initialtemperature ramp in nitrogen (black) peaks distinctly at 100° C. inagreement with the hypothesis that the initial weight loss involves theloss of adventitious absorbates. When the reduced catalyst is oxidizedby treatment with CO₂ (red traces), two separate events are observed tooccur, the first at approximately 650° C. and the second occurring atapproximately 720° C. The bimodal distribution for weight change underoxidizing conditions is reproducible in both CO₂ treatment steps. Theseobservations are consistent with oxygen abstraction from CO₂ occurringat two different sites, one active at slightly lower temperature thanthe other. In the reduction steps (dashed line), a bimodal distributionis also observed. A low temperature weight change is observed atapproximately 400° C. and is minor compared to the higher temperatureweight change observed. Catalyst oxidation by O₂ (air)(-- -) occurs atlower temperatures (˜100-400° C.) compared to CO₂ (˜650-750° C.). Thisalso shows the relative strengths of O₂ and CO₂ as oxidants and affinityof the catalyst for O₂ relative to CO₂.

In summary, the AutoChem-MS studies using isotopically labeled C¹⁸O₂yield strong evidence in support of the hypothesis thatFe₂O₃(SnO₂)_(1.41)(Al₂O₃)_(1.82) removes oxygen from CO₂ and transfersit to other carbon sources. The appearance of C¹⁸O and C¹⁶O¹⁸O duringoxidation of the reduced catalyst with C¹⁸O₂ shows the capability of thecatalyst to abstract oxygen from carbon dioxide as well as the abilityto transfer catalyst-ligated oxygen to an external carbon source. Theappearance of C¹⁶O¹⁸O, C¹⁸O, and C¹⁸O² during reduction of the ¹⁸Olabeled oxidized catalyst shows the ability of the catalyst to transferligated oxygen's to carbon sources. It is clear that in addition to thetransformations which occur on the desired reaction pathway, numerousother transformations occur in side routes on the same time scale. FIG.10 depicts a mechanism which could account for the events observed underthe experimental conditions used in the mass spectroscopy study.Starting from top and moving clockwise, the catalyst precursor isactivated by reduction with CO producing CO₂ and vacancies in thecoordination sphere of the active site. The active sites are occupied byoxygen of CO₂ and CO is produced (top right corner). Oxygen from CO₂ iscombined with CO to make CO₂ again and regenerate coordinativelyunsaturated reactive metal centers. The coordinatively unsaturated metalcenters can also bind CO through the nucleophilic carbonyl carbon, andat this point a series of reversible insertions can be postulated toaccount for the observed oxygen scrambling. One skilled in the art maybe able to use knowledge of these mechanisms and optimize the processfor CO₂ utilization via conversion to CO accordingly.

6.4. Tin/Iron Oxide Larger Scale Demonstration

Demonstration of Production of Carbon Monoxide from Carbon Dioxide and aSolid Carbon Source

A bench-scale fluidized bed reactor was used to demonstrate theformation of CO using CO₂, a solid carbon source, and a promotedcatalyst. The fluidized bed reactor consists of a ¾ inch in diameterstainless steel pipe 5 inches long with a disengagement zone thatexpands to 1.5 inches in diameter. A stainless steel frit is used tohold up the catalyst bed and solid carbon source particles. In thisstudy, SnO₂Al₂O₃(Fe₂O₃)₃ promoted with K and Mg was used as the catalystand pet coke char was used as the solid carbon source. The pet coke wastreated at 800° C. for 6 hours in a nitrogen purge to produce the petcoke char. Catalyst and pet coke char particles were mixed together andloaded into the reactor. The reactor is heated to reaction temperature,typically 800° C., in a nitrogen purge. The reaction is initiated bydirecting a CO₂ stream to the fluidized bed and product gases aremeasured using a CO/CO₂ analyzer. The product stream from the reactor isdiluted with a 200 sccm nitrogen stream before the analyzer to maintainthe minimum flow required for the analyzer.

Elementary reaction experiments were also performed much like in the TGAto observe each step in the proposed mechanism on a larger scale. Ineach of these experiment steps, the catalyst (and solid carbon source inthe fourth step) was heated in the gas specified in Table 3. The firststep is a temperature ramp to 800° C. in N₂ to desorb any gas speciesfrom the surface of the catalyst. Step 2 is a temperature ramp to 800°C. in 10% CO to reduce the catalyst as proposed in the mechanism. Step 3is a temperature ramp to 800° C. in pure CO₂ to observe if the catalystcan be oxidized by the CO₂ to form CO. Step 4 is a temperature ramp to800° C. with the presence of pet coke char in N₂ to observe CO formationusing the oxygen stripped from the CO₂ and the carbon in the pet cokechar to form CO. Table 3 shows the conditions for each elementaryreaction step.

TABLE 3 Reaction conditions for each step in the fluidized-bed reactorTemperature N₂ ramp Hold Flow Product rate Temp. rate Dilution Step (°C./min) (° C.) Gas (SCCM) (SCCM) 1 10 800 N₂ 100 200 2 10 800 10% CO 100200 in N₂ 3 10 800 CO₂ 100 200 4* 10 800 N₂ 100 200 *Note: Pet coke charwas added to the catalyst before Step 4

Bench-Scale Fluidized Bed Reactor Results

The following results describe the observations seen from the elementarystep reactions performed in the fluidized bed reactor outlined in Table3. It is important to note that the CO and CO₂ vol % profiles shown inthe Figures below include a 200 SCCM (˜66% of total flow rate) dilutionstream required by the analyzer.

FIG. 11 shows the temperature, CO, and CO₂ profiles during thetemperature ramp, hold, and cool-down of elementary reaction step 1 inN₂. CO and CO₂ begin to desorb from the surface of the catalyst duringthe temperature ramp and their concentrations in the gas phase begin topeak near the hold temperature of 800° C.

FIG. 12 shows the CO and CO₂ profiles as a function of temperature.During the temperature ramp, CO₂ begins to desorb around 400° C. andreaches a peak of 0.2 vol % at 750° C. Then, the CO₂ concentration dropsoff near 0 vol % in the first 20 minutes of the hold time. CO is alsoobserved desorbing in the 700 to 800° C. range, but at very low levelsin the ppm range. These observations confirm the CO₂ and CO desorptionseen in the TGA experiments. They differ somewhat in the specifictemperature at which they are observed compared to TGA and MS results.This could be due to the presence of potassium and magnesium promoterson the surface of the catalyst materials used in the fluidized bedreactor, which could cause CO₂ to be absorbed as carbonates.

FIG. 13 shows the temperature, CO, and CO₂ profiles during thetemperature ramp and hold of elementary reaction step 2 in 10% CO withthe remaining balance N₂. The CO concentration begins to decrease whileCO₂ is produced and begins to increase throughout the temperature rampindicating that the CO is reducing the catalyst to form CO₂. Then, oncethe hold temperature is reached, the CO₂ gradually drops off while COgradually increases over the next 400 minutes.

FIG. 14 shows the CO and CO₂ profiles as a function of temperature. CO₂concentration peaks near 3.5% at 750° C. and begins a slow gradualdecrease over the next 400 minutes. The amount of CO₂ produced is morethan expected from just reduction of the catalyst and may indicate thatother reactions may be occurring. One possibility is that the forwardBouduard reaction is consuming the CO to form CO₂ and C deposits on thesurface of the catalyst. Additional evidence could be obtained from MSexperiments performed with isotopic CO and CO₂ to discern thispossibility.

FIG. 15 shows the temperature, CO, and CO₂ profiles during thetemperature ramp, hold, and cool-down of elementary reaction step 3 inpure CO₂. CO is produced during the temperature ramp and begins todecrease near the hold temp of 800° C. About 40 minutes after thetemperature hold, the CO₂ concentration begins decreasing, as well asthe CO, due to a pressure build up before the stainless steel fritholding up the catalyst bed. As a safety measure, the feed gas, CO₂, wasvented through pressure relief valves and the feed gas flow rate to thecatalyst bed was greatly reduced resulting in the CO₂ concentrationdropping off. However, it is still observed that CO₂ was consumed and COproduced indicating that the catalyst was being oxidized by the CO₂ toform CO.

FIG. 16 shows the CO and CO₂ profiles as a function of temperature. COis produced near 400° C. and increases to a peak of 15% near 700° C. andbegins decreasing for the next 40 minutes of the hold section.

FIG. 17 shows the temperature, CO, and CO₂ profiles during thetemperature ramp, hold, and cool-down of elementary reaction step 4 inN₂. Pet coke char was added to the catalyst bed at ambient temperaturebefore the ramp while maintaining an N₂ purge to prevent air fromentering the reactor and contacting the catalyst. CO and CO₂ areproduced in a short amount of time near the end of the temperature rampsection.

FIG. 18 shows the CO and CO₂ profiles as a function of temperature. CO₂was observed at low levels from 200 to 600° C. during the temperatureramp and began to sharply increase to a peak of 1.7% near 800° C. beforequickly dropping off to low levels 30 min after reaching 800° C. CO wasalso produced and followed a similar profile peaking just above 0.5%.The observation of CO shows the formation of C—O bonds between pet cokeand oxygenated catalyst while the observation of a high percentage ofCO₂ indicates that scrambling mechanisms are definitely occurring on thesame time scale as the CO dissociation. Further, it may indicate thatthe rate limiting step to the mechanism could be dissociation of CO fromthe active site of the catalyst. In a CO₂ dilute environment the rate ofCO dissociation from the active site should be slower if the rate law isdirectly dependent on the concentration of CO₂ (needed to displace COfrom the site and reoxidize the catalyst).

It is also noteworthy that the amount of CO₂ observed during this stepis somewhat high and unexpected between ˜200-600° C. It is possible thatsome CO₂ from step 3 may have adsorbed without conversion to CO in step3. Then, in a CO₂ dilute environment the CO₂ could desorb. However, ifit were truly simple physical adsorption, all CO₂ would be anticipatedto be purged away well before the temperature reaches 200° C. Theobservation of approximately 0.25 vol % CO₂ above 200° C. could beexplained by any of several reactions of the various oxides with carbonto produce CO₂. For example, SnO₂ is thermodynamically favored to bereduced by carbon to make carbon monoxide and is likely also befavorable for CO₂ formation. The increase observed near 800° C. is infirm agreement with thermodynamic calculations and is likely theinvolvement of sites which are harder to reduce. Very little CO wasproduced in step 4 relative to CO₂, again consistent with explanationsinvolving a reduced rate of CO displacement and more extensive oxidationof pet coke.

6.5. Ruthenium/Iron Oxide Carbon Dioxide Utilization

Dry hydrocarbon reforming is the process of converting C_(x)H_(2x+2) andCO₂ to syngas containing CO and H₂, typically including some H₂O andCO₂. The conversion approaches 100% near 800° C.

A few pivotal papers for the field were published in the early 1990's byAshcroft et al.[1] and researchers from Haldor Topsoe.[2]. TheHaldorTopsoe work included numerous transition metals on MgO support,one being Ru. [2] Several researchers have investigated ruthenium-basedsystems for dry methane reforming since. In 1999, Matsui et al.investigated 5 wt % Ru on La₂O₃, Y₂O₃, ZrO₂ and Al₂O₃ at 600° C. andapproximately 1 atm CO₂ and CH₄ pressures finding that CO₂ and methaneare readily converted to synthesis gas on La₂O₃, Y₂O₃, and Al₂O₃supports.[3]. Near that time Bradford et al. reported ruthenium (0.5-5%)on Al₂O₃, TiO₂, and carbon and tested low pressure streams (0.225CO₂,0.225CH₄, 0.55He) observing 11-12% CO₂ conversions at 450° C.[4]Crisafulli et al. took the approach of impregnating nickel catalystswith ruthenium to improve the performance for dry methane reforming.[5]Nickel (˜2%) supported on SiO₂ and H-ZSMS was impregnated with Ru(0.1-0.6%) and showed reforming of methane (0.15 atm CH₄, 0.15 atm CO₂)at 600° C. to improve with increasing Ru concentration. A perovskiteformulation was studied, CaRuO₃[6] as well as a mixed-metal perovskitesof lanthanides (La, Sm, Nd) with Ru—Ni co-catalysts(Ln_(1-x)Ca_(x)Ru_(0.8)Ni_(0.2)O₃)[7]. The perovskites showed highconversion of CH₄ and CO₂ to CO at 700° C. and 800° C. at 1 atm totalpressure. Ruthenium was investigated on Al₂O₃ and SiO₂ at 1 wt %loadings [8], where in a dilute gas mixture (0.1 atm CH₄, 0.1 atm CO₂,0.8 atm helium) at 550° C., the methane conversions are 12-14% increasedto 52-57% at 750° C. Sutton et al. also probed 1 wt % Ru on Al₂O₃ fordry methane reforming applied to biomass gasification More recently,Haldor Topsoe has reported on Ru supported on ZrO₂ at low pressures(˜0.21 bar CH₄: 0.83 bar CO₂, 1.3 bar total pressure)[10] while othershave reported on a combined partial methane oxidation/carbon dioxidereforming application at low temperature (550° C.) with 8 wt % rutheniumon A1203 doped with cerium.[11]

To our knowledge, no one has reported co-catalyst formulations ofruthenium and iron for dry hydrocarbon reforming or dry methanereforming or for the application of the catalyzed dry reforming reactionto any synthesis gas process, such as integrated gasification combinedcycle (IGCC), biomass gasification, or Fischer-Tropsch synthesis ofliquid transportation fuels. Ru—Zr—Fe metal alloys (approximately equalpercentages of each metal) have been reported for the methanation of CO₂at 100° C. using H₂ (hydrogenation) but not for syngas via dry methanereforming.[12]

We recently discovered that mixed metal oxides of iron and small amountsof ruthenium (˜0.5-1.5 wt %) can be formulated by standardco-precipitation methods, and that mixed metal oxides thereof catalyzethe dry reforming of methane utilizing CO₂ as the oxygen source (drymethane reforming). Mechanistic investigation indicates that oxygenspillover occurs, whereby in separate steps, we observe that thefully-reduced catalyst begins to react with pure CO₂ at approximately400° C., increasing in mass until the weight equals the weight of theoxidized Ru—Fe starting material. Exposure of this oxidized catalyst topure methane shows weight loss of similar magnitude with concomitantproduction of CO and H₂ (FIGS. 19-20). The onset of this reactivity isbetween 500 and 600° C. with apex in activity at 600° C. This activityhas been observed in separate reaction steps with the aid ofthermogravimetric analysis and mass spectroscopy, shown below. Reductionwith CO results in a 22% weight loss which looks fully reversible whenoxidized with CO₂. The magnitude is indicative of oxygen spilloverbetween Fe and Ru since Ru is present in only a small, catalytic, amountrelative to Fe₂O₃. When the reoxidized catalyst was purged with CH₄, itagain showed a significant weight loss, even higher than the weight lossobserved during reduction with CO. Even more promising, the predominantproducts as indicated by MS are CO and H₂, with only a small amount ofCO₂ detected. Under these conditions, this catalyst appears promisingfor further development.

We have also observed that the material is active in a fixed-bed reactorsystem under a co-feed of CO₂ and CH₄ and confirmed the formation of COand H₂. Results are provided below (FIGS. 21-24). Highlights are that atapproximately 600° C. and 25 bar of a feed mixture (CH₄: CO₂=6:1), weobserved production of approximately 10 vol % CO and 10 vol % hydrogen.In terms of CO₂ conversion we observed approximately 30% conversion.When the total pressure is reduced to approximately 1 bar, the CO₂conversion is approximately 16%. When the feed composition is changed toa CO₂:CH₄ ratio of 1:1 and 1 bar total pressure, approximately 2 vol %CO is formed (˜3% CO₂ conversion) at 500° C., whereas at 600° C.,approximately 5 vol % CO and 1 vol % H₂ are made (approximately 12% CO₂conversion). When the temperature is raised to 800 C approximately 30volume percent CO is formed and 10 volume percent hydrogen. See FIG. 24showing the performance of the Ru—Fe catalyst at 630° C. and 780° C.with a total of 1 atm natural gas and carbon dioxide mixture (1 molCH₄:1 mol CO₂). In all cases when the total pressure is increased to 50bar, no synthesis gas products are observed.

At this point the CO₂-methane reforming catalyst formulation could beincorporated into a process by modifying the catalyst formulation toinclude an additional phase capable of forming a target product from thesynthesis gas made by the Ru-Fe phase. Such products are methanol orFischer-Tropsch fuels. For methanol synthesis, the approach would be todevelop a catalyst with a copper component similar to the copper-zincaluminate catalyst used for commercial methanol synthesis. Thecopper-zinc aluminate could be incorporated as an additional phase tothe current dry methane reforming formulation, with the goal being torun the process in a single reactor using a single bifunctional catalystmaterial. However, it does not necessarily have to be done this way, andin fact, since the process conditions which we have currently observedfor our dry methane reforming catalyst are lower in pressure and higherin temperature than the conditions currently encountered in methanolsynthesis from syngas, the accomplishment may be difficult to achieve.To avoid this difficulty, we could set the process up in two reactionzones, with the synthesis gas produced from CO₂ and CH₄ being fed to amethanol synthesis zone. The dry methane reforming catalyst and themethanol synthesis catalyst would be kept separate.

Fischer-Tropsch fuels from CO₂-derived synthesis gas is another processwhich could incorporate the new Ru—Fe catalyst. Like the methanolapproach, the objective is conversion of the syngas to liquid fuels, butin this case it makes a little more sense to consider a single catalystapproach. The current formulation contains components which are known tohave FT-activity and we currently know that synthesis gas can beproduced at pressures which could be used in high-temperature FTprocesses. In one embodiment, the temperature of the CO₂-derivedsynthesis gas is lowered by few hundred degrees, while moving thesynthesis gas from the CO₂-utilization zone to the FT-zone, where itwill be converted to transportation fuels.

The ruthenium-iron mixed metal oxide can be prepared by the followingpreparation. For the preparation of approximately 2.00 g Ru_(0.01)FeO₁₅₂, 2.00 g of ruthenium nitrosyl nitrate (Strem Chemicals, 1.5% Ru)and 10.01 g of iron (III) nitrate nonahydrate (Sigma Aldrich, ≧98%) weredissolved into 100.70 g of deionized water. The pH of the solution was1.23. 34.55 g of 9.07 wt % NaOH solution was added drop-wise whilemixing on a stir plate to reach pH 7.56. The solids were collected viavacuum filtration and then washed with 1 L of deionized water. The pH ofthe final 25 mL of wash filtrate was ˜6.5 by pH strip. The wet cake,12.61 g, was dried overnight at 120° C. and then calcined at 650° C. for2 hours after a ramp up at 3° C./min A total of 1.93 g was collected, a95.5% yield. Elemental analysis by ICP-MS showed Ru 1.2%, Fe 71.9%,theory Ru 1.2%, Fe 68.8%.

6.6. References for Section 6.5 (Ruthenium Oxide Iron Oxide)

-   1. Ashcroft, A. T.; Cheetham, A. K.; Green, M. L. H.; Vernon, P. D.    F., PARTIAL OXIDATION OF METHANE TO SYNTHESIS GAS-USING    CARBON-DIOXIDE. Nature 1991, 352, (6332), 225-226.-   2. Rostrup-Nielsen, J. R.; Hansen, J.-H. B., Journal of Catalysis    1993, 144, 38.-   3. Matsui, N.; Anzai, K.; Akamatsu, N.; Nakagawa, K.; Ikenaga, N.;    Suzuki, T., Reaction mechanisms of carbon dioxide reforming of    methane with Ru-loaded lanthanum oxide catalyst. Applied Catalysis    a-General 1999, 179, (1-2), 247-256.-   4. Bradford, M. C. J.; Vannice, M. A., CO2 reforming of CH4 over    supported Ru catalysts. Journal of Catalysis 1999, 183, (1), 69-75.-   5. Crisafulli, C.; Scire, S.; Minico, S.; Solarino, L., Ni—Ru    bimetallic catalysts for the CO2 reforming of methane. Applied    Catalysis a-General 2002, 225, (1-2), 1-9.-   6. Reller, A.; Davoodabady, G.; Portmann, A.; Oswald, H. R. In The    8th European COngress on Electron Microscopy, Budapest, 1984;    Budapest, 1984.-   7. Goldwasser, M. R.; Rivas, M. E.; Pietri, E.; Perez-Zurita, M. J.;    Cubeiro, M. L.; Gingembre, L.; Leclercq, L.; Leclercq, G.,    Perovskites as catalysts precursors: CO2 reforming of CH4 on    Ln(1-x)Ca(x)Ru(0.8)Ni(0.2)O(3) (Ln=La, Sm, Nd). Applied Catalysis    a-General 2003, 255, (1), 45-57.-   8. Ferreira-Aparicio, P.; Rodriguez-Ramos, I.; Anderson, J. A.;    Guerrero-Ruiz, A., Mechanistic aspects of the dry reforming of    methane over ruthenium catalysts. Applied Catalysis a-General 2000,    202, (2), 183-196.-   9. Sutton, D.; Parle, S. M.; Ross, J. R. H., The CO2 reforming of    the hydrocarbons present in a model gas stream over selected    catalysts. Fuel Processing Technology 2002, 75, (1), 45-53.-   10. Jakobsen, J. G.; Jorgensen, T. L.; Chorkendorff, I.; Sehested,    J., Steam and CO2 reforming of methane over a Ru/ZrO2 catalyst.    Applied Catalysis a-General 2010, 377, (1-2), 158-166.-   11. Ji, H.; Feng, D.; He, Y., Low-temperature utilization of CO2 and    CH4 by combining partial oxidation with reforming of methane over    Ru-based catalysts. Journal of Natural Gas Chemistry 2010, 19, (6),    575-582.-   12. Tada, T.; Habazaki, H.; Akiyama, E.; Kawashima, A.; Asami, K.;    Hashimoto, K., AMORPHOUS FE-VALVE METAL-PT GROUP METAL ALLOY    CATALYSTS FOR METHANATION OF CO2. Mater. Sci. Eng. A-Struct. Mater.    Prop. Microstruct. Process. 1994, 182, 1133-1136.

It is to be understood that, while the invention has been described inconjunction with the detailed description, thereof, the foregoingdescription is intended to illustrate and not limit the scope of theinvention. Other aspects, advantages, and modifications of the inventionare within the scope of the claims set forth below. All publications,patents, and patent applications cited in this specification are hereinincorporated by reference as if each individual publication or patentapplication was specifically and individually indicated to beincorporated by reference.

1. A mixed transition metal iron (II/III) catalyst for catalyzing CO₂oxidation of carbon or an organic compound.
 2. The mixed transitionmetal iron (II/III) catalyst of claim 1, wherein the mixed transitionmetal iron (II/III) catalyst comprises an iron (II/III) and a secondmetal selected from the group consisting of Ag, Bi, Co, Cu, La, Mn, Sn,Ru, and Zn.
 3. The mixed transition metal iron (II/III) catalyst ofclaim 1, further comprising a support.
 4. The mixed transition metaliron (II/III) catalyst of claim 1, further comprising an alkali oralkaline-earth element promoter.
 5. The mixed transition metal iron(II/III) catalyst of claim 3, wherein the support is Al₂O₃, SiO₂, TiO₂,ZrO₂ or a mixture thereof.
 6. The mixed transition metal iron (II/III)catalyst of claim 5, having the formulaFe₂O₃(SnO₂)_(0.1-10)(Al₂O₃)_(0.1-10).
 7. The mixed transition metal iron(II/III) catalyst of claim 6, having the formulaFe₂O₃(SnO₂)_(1.0-3.0)(Al₂O₃)_(1.0-3.0).
 8. The mixed transition metaliron (II/III) catalyst of claim 2, having the formula(RuO₂)_(0.001-0.2)Fe₂O₃.
 9. The mixed transition metal iron (II/III)catalyst of claim 2, having the formula (RuO₂)_(0.005-0.05)Fe₂O₃.
 10. Amethod for converting CO₂ and carbon to carbon monoxide which comprisescontacting the mixed transition metal iron (II/III) catalyst of claim 1with an appropriate CO₂ feed stream under appropriate temperature andpressure conditions.
 11. The method of claim 10, wherein the carbon, themixed transition metal iron (II/III) catalyst, and the appropriate CO₂feed stream are reacted together at the same time.
 12. The method ofclaim 11, wherein the carbon, the mixed transition metal iron (II/III)catalyst, and the appropriate CO₂ feed stream are reacted together in afluidized bed. 13.-24. (canceled)